Above ground tanks are used extensively for storage of many types of fluids. Among those are storage applications for a range of fluids in the oil and gas industry. Many of these tanks are in excess of 50 years old and corrosion, outdated construction techniques and structural problems result in leaks which may not be easily detectable and are very difficult to locate.
Leaks in the side of tanks are clearly visible, thus, easily detectable and locatable. Large leaks in the bottom of tanks, while possibly not visible, can be detected by measuring the change of the volume of oil in the tank. For example, leaks on the order of 100 gallons per hour can be detected by measuring the volume change over a 24 hour period.
Small leaks through the bottom of tanks cannot be easily detected. Oil does not always seep at or to the edges of such tanks, but moves directly into the ground and may migrate vertically downwards thus leaving no visual indication of such a leak. Furthermore, volumetric changes due to such small leaks are not generally detectable because they are masked by small volumetric changes which occur due to external temperature changes and planned input to and output from such tanks during the measurement period. Therefore, leaks on the order of approximately one gallon per hour are currently undetectable and cannot be located. These small leaks represent an environmental problem as well as a significant loss in revenue.
The technology exists to repair such leaks. For example, above ground leaks can be repaired using patches or other means. Furthermore, the technology exists to repair leaks below ground level and to provide additional safeguards for the future such as using double bottom systems to ensure that if a leak does occur it is detectable and that the oil does not contaminate the ground. However, the below ground technology is expensive and a complete upgrade of all tanks will take the industry several years to accomplish. Therefore, a method is required to identify the problem tanks so that they can be given priority.
Past work in this area has found that such leaks continuously generate a very low level acoustic signal. Calculations indicate that energy release is on the order of 0.5 watts. The signals have a general pattern and are fixed in location because they emanate from the leak. Current methods try to use directional listening devices, and location determination by methods of crossed bearings or triangulation. These methods require that the emission signal be larger than the background noise. This is seldom true, requiring long listening times to gain a statistically significant number of estimates. Many of the larger energy arrivals come from outside of the tank requiring methods to discriminate against false positions. Relying on the larger signal events for location makes the existing methods very susceptible to multipathing and reflections within these very complex structures. Further compounding the problems of direct event location is the large amount of information. With sample rates on the order of 50,000 samples per second, on the order of 20 gigabytes would need to be recorded for one channel in post processing of a 24 hour period.
A need still exists for an effective method and apparatus that uses basically real-time data to acoustically detect and locate small leaks in above ground storage tanks in the presence of high ambient noise generated in or outside of the tank.